Ch 3

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Ch 3 Pressure, Altitude Density
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Ch 3 Pressure, Altitude Density

• Introduction
• Pressure and its variations have important
  applications for aviation, ranging from
  measurements of altitude and airspeed to the
  prediction of winds and weather.
• This chapter focuses on several of these
  applications.
• When you complete the chapter, you will have a
  good physical understanding of atmospheric
  pressure, altimetry, and density altitude
  (Lester, 2006).

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Ch 3 Pressure, Altitude Density

• Introduction
• You will develop important background knowledge
  about the global patterns of atmospheric
  pressure.
• This information will prove useful in the next
  chapter when we examine the causes and
  characteristics of atmospheric winds (Lester,
  2006).

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Ch 3 Pressure, Altitude Density

• Section A Atmospheric Pressure
• Pressure Measurements
• Section B Charting Atmospheric Pressure
• Station and Sea Level Pressure
• Sea Level Pressure Patterns
• Constant Pressure Charts
• Section C The Pressure Altimeter
• METARs and Altimeter Settings
• Section D - Density

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Ch 3 Pressure, Altitude Density

• Section A Atmospheric Pressure may also be
  defined as the weight of a column of the
  atmosphere with a given cross-sectional area
  figure 3-1
• Atmospheric Pressure Weight (Force) / Area
  14.7 lbs/1 in sq 14.7 lbs/in sq
• Hydrostatic balance figure 3-2 the balance
  between the downward-directed gravitational force
  and an upward-directed force caused by the
  decrease of atmospheric pressure with altitude is
  called hydrostatic balance

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Ch 3 Pressure, Altitude Density

• Pressure Measurements
• Mercurial barometer one of the most basic
  devices for the measurement of pressure
• Aneroid barometer another pressure instrument,
  known as the aneroid barometer is more frequently
  used outside the laboratory
• has no liquid
• operates on differences in air pressure between
  the atmosphere and a closed vessel (an aneroid
  cell)
• aneroid means not wet Figure 3-4

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Ch 3 Pressure, Altitude Density

• Section B Charting Atmospheric Pressure
• Station and Sea Level Pressure surface pressure
  measurements are most useful if they can be
  compared with nearby measurements at the same
  altitude
• over land areas, the direct comparison of station
  pressures are usually difficult because weather
  stations are often at different altitudes Figure
  3-5
• the atmospheric pressure measured or estimated at
  an elevation equal to mean sea level

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Ch 3 Pressure, Altitude Density

• this extrapolated sea level pressure (station
  pressure corrected for elevation) is used by
  pilots to determine altitude
• also used in aviation reports to depict the
  atmospheric pressure of a reporting location

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Ch 3 Pressure, Altitude Density

• Sea Level Pressure Patterns
• Surface analysis charts a chart which shows
  pressure as well as other meteorological
  conditions at the surface of the earth
• Isobars lines of constant pressure figure 3-6
• High (H) location where the sea level pressure
  is high compared to its surroundings
• Ridge an elongated region of relatively high
  pressure

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Ch 3 Pressure, Altitude Density

• Low (L) a roughly circular area with a lower
  sea level pressure in the center as compared to
  the surrounding region
• Trough an elongated region of relatively low
  pressure these features are to a surface
  analysis chart what mountains and valleys are to
  a topographical chart

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Ch 3 Pressure, Altitude Density

• Pressure gradient a difference in pressure over
  a given distance
• Semi-permanent pressure systems Bermuda High,
  Aleutian Low, Siberian High, Icelandic Low,
  Pacific High

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Ch 3 Pressure, Altitude Density

• On a surface analysis chart, the solid lines
  that depict sea-level pressure patterns are
  called isobars

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Ch 3 Pressure, Altitude Density

• On a surface analysis chart, close spacing of
  the isobars indicates strong pressure gradient

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Ch 3 Pressure, Altitude Density

• Constant Pressure Charts a constant pressure
  surface is one where the pressure is the same at
  all points
• like the oceans surface, a constant pressure
  surface is not necessarily level
• many upper air weather charts that you may use
  are called constant pressure analysis charts or
  simply constant pressure charts figure 3-7
• the interpretation of a constant pressure chart
  is identical with the sea level pressure chart as
  far as highs, lows, troughs, ridges and gradients
  are concerned

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Ch 3 Pressure, Altitude Density

• the main difference is one of terms used to
  describe the elements
• on constant pressure surfaces, lines of constant
  height are called contours rather than isobars
• gradients are height gradients rather than
  pressure gradients figure 3-8

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Ch 3 Pressure, Altitude Density

• Pressure Altitude the altitude of a given
  pressure surface in the standard atmosphere
  figure 3-9
• pilots can determine pressure altitude by setting
  the standard sea level pressure, 29.92 inches Hg
  in the aircraft altimeter

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Ch 3 Pressure, Altitude Density

• Pressure altitude is the altitude indicated
  when the altimeter setting is 29.92.
• If you set your altimeter at 29.92 and fly at
  18,289 feet indicated altitude, you will be
  flying along the 500 mb constant pressure surface

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Ch 3 Pressure, Altitude Density

• Section C The Pressure Altimeter the
  altimeter is essentially an aneroid barometer
  that reads in units of altitude rather than
  pressure
• this is possible by using the standard atmosphere
  to make the conversion from pressure to altitude
  Appendix B Figure 3-10
• Indicated altitude the altitude measured by
  your altimeter
• True altitude the actual altitude of your
  aircraft above mean sea level
• Absolute altitude the altitude of your aircraft
  above the ground

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Ch 3 Pressure, Altitude Density

• Altimeter errors there are three specific
  altimeter errors caused by nonstandard
  atmospheric conditions
• 1. sea level pressure different from 29.92 inches
  of mercury
• 2. temperature warmer or colder than standard
  temperature
• atmospheric pressure decreases with altitude more
  rapidly in cold air than in warm air
• 3. strong vertical gusts

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Ch 3 Pressure, Altitude Density

• Altimeter setting is the value to which the
  barometric pressure scale on the altimeter is set
  so the altitude indicates true altitude at field
  elevation

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Ch 3 Pressure, Altitude Density

• On warm days pressure surfaces are raised and
  the indicated altitude is lower than true altitude

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Ch 3 Pressure, Altitude Density

• Altimeter setting figure 3-10 variable sea
  level pressure is usually taken care of by
  adjusting the altimeter to the proper altimeter
  setting
• this is the sea level pressure determined from
  the station pressure and the standard atmosphere

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Ch 3 Pressure, Altitude Density

• METARs and Altimeter Settings
• Aviation Routine Weather Report (METAR)
  altimeter settings for airports worldwide are
  reported and transmitted regularly with other
  weather information in a standard coded format
• these reports are commonly available to pilots
  figure 3-13

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Ch 3 Pressure, Altitude Density

• Type of Report
• Aviation selected special weather report (SPECI)
  the special METAR weather observation is an
  unscheduled report indicating a significant
  change in one or more elements
• Station Designator and Date/Time of Report each
  reporting station is listed by its four-letter
  International Civil Aviation Organization (ICAO)
  identifier
• in the contiguous 48 states, the letter K
  prefixes the three-letter domestic location
  identifier

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Ch 3 Pressure, Altitude Density

• Modifier when a METAR is created by a totally
  automated weather observation station, the
  modifier AUTO will follow the date/time element
• RMK A02 indicates the weather observing equipment
  used has the capability of distinguishing
  precipitation type
• the modifier COR is used to indicate a corrected
  METAR which replaces a previously disseminated
  report
• no modifier indicates a manual station or manual
  input at an automated station

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Ch 3 Pressure, Altitude Density

• Temperature and Dew-point the current air
  temperature and dew-point are reported in
  two-digit form in degrees Celsius and are
  separated by a slash
• Altimeter reported in inches of mercury in a
  four digit group without the decimal point and is
  prefixed by an A

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Ch 3 Pressure, Altitude Density

• Remarks the remarks section begins with RMK
• the remark SLP134 refers to the sea level
  pressure of 1013.4 hecto Pascals (hPa)
• the leading 9 or 10 is omitted
• prefix the number with a 9 or 10
• whichever brings it closer to 1,000.0
• a remark such as T00081016 refers to the
  temperature and dew point in tenths degrees
  Celsius
• in this example, the first zero in the sequence
  indicates a plus value for temperature (.8C) and
  the leading one in the sequence shows a minus
  value for dew point (-1.6 C).

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Ch 3 Pressure, Altitude Density

• Section D Density aircraft performance
  depends critically on atmospheric density

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Ch 3 Pressure, Altitude Density

• Pressure altitude and density altitude have
  the same value at standard temperature in the
  standard atmosphere

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Ch 3 Pressure, Altitude Density

• Density Altitude the altitude above mean sea
  level at which a given atmospheric density occurs
  in the standard atmosphere
• can be interpreted as pressure altitude corrected
  for non-standard temperature differences
• in warmer than standard surface conditions, you
  would say that the density altitude is high
  that is, operation of your aircraft in a high
  density altitude condition is equivalent to
  taking off from a higher airport during standard
  conditions
• in a high density altitude situation, the actual
  density at the surface is found above the airport
  elevation in the standard atmosphere figure 3-14
  

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Summary

• Atmospheric pressure is an essential component of
  aviation weather basics.
• An understanding of pressure is the foundation
  for understanding such diverse and important
  topics as altimetry, winds, and storms.
• In this chapter, you have learned about the
  useful relationship between atmospheric pressure
  and the weight of the atmosphere and how that
  relationship allows us to measure pressure and
  altitude (Lester, 2006).

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Summary

• Details about the distribution of average sea
  level pressure around the globe, as well as the
  terminology and methods for the interpretation of
  atmospheric pressure charts at the surface and
  aloft should now be part of your growing
  knowledge of aviation weather (Lester, 2006).

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Summary

• You have gained valuable insight into the effects
  of atmospheric variations in pressure and
  temperature on the accuracy of pressure altimeter
  measurements.
• You have been introduced to standard weather
  reports available to pilots.
• In particular, you have learned where to find
  locations of the reporting stations, times,
  pressures, temperatures, and altimeter settings
  in those reports.

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Summary

• Finally, you have become familiar with the
  concept of density altitude and its impact on
  aircraft performance (Lester, 2006).